1. Field of the Invention
The subject-matter of the present invention relates to a new method for coloring ceramic surfaces by using the aqueous solution from a gold compound.
2. Description of the Related Art
The production of pink color shades has proven to be relatively problematic for the ceramic industry. It is standard practice to produce pigment colors by sintering together the most varied metal oxides and applying these to the ceramic surface with boric acid as fluxing means.
The reference DD 224026, for example, describes a "pink coloring element" with the approximate composition: 2 CaCO.sub.3 .times.SnO.sub.2 .times.2SiO.sub.2 .times.H.sub.3 BO.sub.3 and containing 0.1-1.5% Cr.sub.2 O.sub.3 as color-causing component.
Such pigment powders have the disadvantage that they must either be added to the total ceramic mass, making it impossible to add a pattern, or that they are applied only as a thin layer to the ceramic surface and thus change the ceramic surface, so that a subsequent treatment of the surface is no longer possible.
It is furthermore known that colloidal gold, which is stabilized through tin oxide, results in dark ruby-red to lilac-red color shades (Cassius gold purple). The admixture of this gold purple to glasses and glazes consequently results in correspondingly ruby-red to lilac-colored glasses (gold ruby glass) and glazes (S. Stephanov et al. Ceramic Glazes, Wiesbaden/Berlin 1988, page 127).
It is furthermore known that when gold sulphoresinates, suspended in an organic solvent, are applied to a ceramic surface and are fired at 500-800.degree. C., this results in a thin coating of metallic gold with a typical gold luster (compare DE-C1-41 22 131).
According to the DE-C1 43 20 072, diluted, aqueous solutions of gold salts, particularly gold chlorides such as gold(III)-chloride or tetrachloro gold acid, which can be obtained commercially as monohydrate and trihydrate, can be used to color in the surfaces of ceramic bodies, such as tiles or porcelain. A pink to blue coloration is obtained in a very simple way by calcining the surface of the ceramic body, colored in this way.
The disadvantage of this method is that the gold salt solutions used are not stable under normal storage and processing conditions and react in particular with metallic surfaces in the devices used and with reducing impurities or additives by forming amorphous gold, which is precipitated out as "slurry."
The object of the invention therefore was to find a new method for coloring ceramic surfaces, particularly coloring with pink color shades, which method allows the introduction of color at a later date to the surface layer of the ceramic material and the coloring of this surface layer to a depth of about 0.5-2 mm, so that it is possible to apply a pattern as well as use a subsequent treatment on the surface. A stable, non-toxic gold compound is to be used for the method.